An underground continuous wall work method was employed in around 1950 in Europe as a work method for constructing an earth-retaining structure or a cut-off wall; and in Japan, in 1959, it was introduced for the first time for a cut-off wall for river stop. Afterwards, in construction works in and around urban areas, noise, vibration and neighboring land subsidence to occur in the work with steel sheet piles or precast piles, and the influence of the work on the ground water level are social problems; and the method is employed as an effective work method for such social/environmental problems. Recently, the method has become employed not only for deep underground and large-scale technologies but also for temporary structures and permanent structures. Further, use of the method is expanding not only for concrete structures but also for soil cement underground wall structures.
In such an underground continuous wall work method, a bentonite-base stabilizer is used for keeping the stabilization of the tunnel wall and facilitating sludge removal of drilled earth and sand. However, in the method, the amount of the sludge to be removed is large and, in addition, the sludge mixed with bentonite is difficult to reuse and requires a high cost for sludge disposal; and therefore, any one substitutable for it is desired.
On the other hand, it is known that a suspension of from 15 to 40% by volume of air foams and a suitable amount of water added to the soil under drilling (hereinafter referred to as an air-foam stabilizer) has various functions as a stabilizer, and that the underground continuous wall work method using the air-foam stabilizer does not specifically differ from the usual method. A necessary amount of air foams and water are jetted out through the drill tip of a drilling machine, and are mixed and stirred with the soil under drilling; and the suspension stabilizes the tunnel wall to be an air-foam stabilizer having the function of fluidity, etc., therefore enabling continuous drilling. The air foams in the removed sludge are left in air, or when a defoaming agent is added thereto, they readily disappear, and therefore, their post-treatment is easy. Accordingly, the sludge amount is reduced to at most ½ as compared with that with the bentonite-base stabilizer; and it may be said that the air-foam stabilizer is excellent in view of the environmental load and the economical aspect thereof.
As an example of using air foams in ground drilling, there is known an air-foam shield tunneling method (for example, Non-Patent Reference 1, Patent References 1-2). The air-foam shield tunneling method is a soil pressure shield tunneling method in which air foams are used as the slurry additive and in which air foam addition enhances the fluidity and the water stop security in removal of soil under drilling in a chamber and on a screw conveyor, and prevents adhesion of cohesive soil therein. The air foams in the removed soil spontaneously disappear or are defoamed by a defoaming agent, and therefore the removed soil is restored to the original state before air foam injection, and its post-treatment is easy. However, owing to the soil pressure inside the chamber that contains air foams therein, the mechanism to resist the shield face soil pressure is conceptually shown, but no detailed investigations have been made relating to the face ground stabilization mechanism by air foams themselves.
On the other hand, in the underground continuous wall work method (for example, Patent Reference 3) or the like, the air-foam stabilizer directly participates in the stability of the drilling tunnel wall like the bentonite-base stabilizer; and therefore, as compared with that in the air-foam shield case, the stabilization mechanism and performance of the stabilizer must be investigated in detail. In particular, the main material of the air-foam stabilizer is air foams, and therefore, the function expression mechanism of the stabilizer in the tunnel wall stabilization and the fluidity significantly differs from that of the bentonite-base stabilizer. Further, regarding the environmental load, the two significantly differ from each other in the sludge removal amount and the processing cost.
In the underground continuous wall work method using the bentonite-base stabilizer, the stabilizer is controlled using the stabilizer property control chart based on the measured data of the specific gravity and the funnel viscosity thereof; and many excellent work results have been obtained. Within a predetermined region of the control chart, the bentonite stabilizer keeps good or somewhat good properties; however, in a case where the state changes to the area outside the region, it is said necessary to take some suitable measures of increasing the amount of bentonite or adding a promoter such as CMC or the like.
The air-foam stabilizer has been invented on the basis of the above-mentioned already-existing knowledges and techniques, and is defined as “a stabilizer which is a homogeneous suspension of soil under drilling, air foams and water and which is excellent in tunnel wall stabilization, water-stop performance and fluidity”. However, the physical properties of the air-foam stabilizer are greatly influenced not only by the amount of the foams to be added and the amount of water but also by the grain size and the consistency characteristic of the soil under drilling. Therefore, in underground drilling with the air-foam stabilizer, suitable control indices capable of evaluating their influences, corresponding to the specific gravity and the funnel viscosity of the bentonite stabilizer, must be provided for drilling control of regulating the amount of the foams and water to be added.
At present, however, any technical means relating to the control indication for the drilling work with the air-foam stabilizer is, in fact, as yet not realized.
Non-Patent Reference 1: the Shield Technology Association, “Air-Foam Shield Work Method” (June 2007)
Patent Reference 1: Japanese Patent No. 3124368
Patent Reference 2: Japanese Patent No. 2768104
Patent Reference 3: Japanese Patent No. 3725750
In the above-mentioned background, an object of the present invention is to solve the prior-art problems and clarify the control indices in the drilling work with an air-foam stabilizer, thereby providing a method for regulating an air-foam stabilizer based on these and providing a novel method for ground drilling.